1. Field of the Invention
The present invention relates to a power amplifier included in a personal portable communication device such as a cellular phone or a personal digital assistant (PDA), and more particularly, to a temperature-compensated bias circuit for a power amplifier.
2. Description of the Related Art
With the development of electronic technologies, portable electronic apparatuses are efficiently designed and costly effectively manufactured. The portable electronic apparatuses largely include pagers, cellular phones, music players, calculators, laptop computers, and PDAs. The portable electronic apparatuses generally require DC power and one or more batteries are used as an energy source for supplying DC power.
Wireless portable communication terminals such as mobile handsets or cellular phones are becoming compact and light. Accordingly, the size of a battery occupying a considerable portion of the mobile handset is becoming smaller to fit into the mobile handset that is compact and light. In case of the cellular phone, along with smaller terminal and battery, longer talk time is required. Thus, the life of the battery is an important factor in the mobile communication terminals such as mobile handsets or cellular phones.
The temperature in use of these personal wireless communication apparatuses changes according to a change of a season, the operation of an amplifier, or an operation duration time. Maintaining a particular feature of a power amplifier in spite of the change in temperature is another important factor in determining performance of the terminal.
A bias circuit to compensate for a temperature is needed for a superior amplification operation of a power amplifier in an appropriate operation range in spite of the change in temperature. According to a conventional technology, a circuit as shown in FIG. 1 is used for the bias of a power amplifier.
FIG. 1 is a circuit diagram of a bias circuit of a conventional power amplifier. Referring to FIG. 1, a transistor Q2 is a simplified form of an amplification end of a power amplifier. A transistor Q1 is a bias transistor, or a DC buffer transistor, which provides a bias voltage to a base of the transistor Q2. Since the transistor Q1 compensates for insufficient current applied to the transistor Q2 when a bias voltage Vy is directly input to the base of the transistor Q2, it is referred to as a DC buffer transistor. In FIG. 1, a power voltage Vcc is applied to the transistors Q2 and Q1 while a reference voltage Vref is applied to a resistor Rref of a bias circuit block 200. A collector static operational current of the transistor Q2 is indicated by static operational current IQ.
Prior to the description of the conventional invention, the typical current characteristic of a diode needs to be understood. Those skilled in the art would easily understand that the current characteristic of a typical diode is identical to that according to a base-emitter voltage of a transistor.
FIG. 2 is a graph showing that the characteristic of current according to a voltage between both ends of a diode or the characteristic of current according to the base-emitter voltage of a transistor, with a parameter of temperature. In FIG. 2, as the temperature increases, a characteristic curve moves to the left so that a diode turn-on voltage VBE(on) decreases. As it is well known, the movement of the curve has a value of about −2mV/° C. When the bias voltage Vbias is constant, the effective base-emitter voltage is that VBE(eff)=Vbias−VBE(on) so that the current increases.
Next, in the temperature compensation operation of the conventional bias circuit block 200 of FIG. 1, it is assumed that a voltage of a Vy node is designed to be 2.6 V by the resistor Rref and two diodes D1 and D2 at the room temperature of about 25° C. This means that a value of the resistor Rref is set such that the voltage between both ends of each of the two diodes connected in series becomes 1.3 V.
The voltage between the base-emitter of the transistors Q1 and Q2 is 1.3 V like the diodes D1 and D2.
When an operation temperature increases, in the transistors Q1 and Q2, as shown in FIG. 2, the base-emitter turn-on voltage VBE(on) decreases so that the static operational current IQ increases. However, since the diodes D1 and D2 have the same temperature dependency as the transistors Q1 and Q2, the voltage Vy decreases accordingly. The decrease of the voltage Vy means a decrease in the base-emitter voltage of the transistors Q1 and Q2. Also, since the effective voltage VBE(eff) between the base-emitter voltage of the transistors Q1 and Q2 does not change, the static operational current IQ is constant.
When the operation temperature decreases, the base-emitter turn-on voltage VBE(on) of in the transistors Q1 and Q2 increases so that the static operational current IQ decreases. However, since the diodes D1 and D2 have the same temperature dependency as the transistors Q1 and Q2, the voltage Vy increases accordingly. The increase of the voltage Vy means an increase in the base voltage of the transistors Q1 and Q2. Also, since the effective voltage VBE(eff) between the base-emitter voltage of the transistors Q1 and Q2 does not change, the static operational current IQ is constant.
To summarize the above operation, the voltage Vy between both ends of each of the diodes D1 and D2 tracks the base-emitter turn-on voltage of the transistors Q1 and Q2 according to the change in temperature so that the effective voltage VBE(eff) is constantly maintained. Thus, in spite of the change in temperature, the static operational current IQ is contact.
However, practically, when the Vy voltage drops to about 2.4 V, the voltage between both ends of the base-emitter of each of the transistors Q1 and Q2 automatically decreases to about 1.2 V. However, in this case, the static operational current IQ of the transistor Q2 increases greater than the size at the room temperature. This is because the sizes of the transistors Q1 and Q2 driving a large amount of current are much greater than those of the diodes D1 and D2 so that the dependency on temperature is not the same. Thus, it is a problem that the voltage Vy must be less than 2.4 V to perform accurate temperature compensation so that the static operational current IQ of the transistor Q2 is constantly maintained.
When the operation temperature drops lower than the room temperature, the voltage Vy increases by the temperature dependency intrinsic to the diodes D1 and D2. When the voltage Vy increases to about 2.8 V, the voltage between both ends of the base-emitter of each of the transistors Q1 and Q2 automatically increases to about 1.4 V. Accordingly, the static operational current IQ of the transistor Q2 decreases compared to the current at the room temperature. For the same reason in a case in which the temperature increases, in order to perform accurate temperature compensation by which the static operational current IQ of the transistor Q2 is constantly maintained, a problem in which the voltage Vy must be greater than the static operational current IQ occurs. FIG. 3 is a graph showing the static operational current IQ when the temperature compensation function is insufficient due to the above problem in comparison with the static operational current IQ in an ideal state.
A variety of circuit techniques have been developed to solve a problem in which maintaining the static operational current IQ of the transistor Q2 constantly by the temperature compensation function based on the temperature dependency of the diodes D1 and D2 is difficult. One of the circuit techniques is that the voltage between both ends of each of the diodes D1 and D2 connected in series is arbitrarily and appropriately changed according to a change in temperature to provide a more ideal static operational current IQ feature.
Referring to FIG. 4, one of the conventional techniques having an additional temperature compensation function is described. This circuit includes a bias circuit block 200 and an amplifier block 210. In the configuration of the circuit, a transistor 226 shows part of an amplification circuit amplifying an RF signal and a transistor 224 is a DC buffer transistor and a resistor R2 DC-biases a base of the transistor 226.
The bias circuit block 200 has the same elements as the amplifier block 210 to form a current mirror shape. A transistor 220 and a transistor 222 make mirrored pairs with the transistor 224 and the transistor 226, respectively, while a resistor R1 makes a mirrored pair with the resistor R2.
A voltage of a node 234 flows from a base of the transistor 220 via the transistor 222 to ground so that a voltage drop is 2VBE. The resistor R1 is connected to a base node 240 of the transistor 222. A DC reference voltage Vref is connected to one side of a resistor Rref and current flowing between both ends of the resistor Rref is Iref.
When the operation temperature increases, the base-emitter turn-on voltage VBE(on) of the transistor 222 decreases. However, since current Imir is almost constantly maintained, the voltage of the node 240 is almost constantly maintained. Thus, an effective voltage between the base-emitter of the transistor 222 increases so that collector current of the transistor 222 increases and the voltage of the node 234 drops. When the voltage of the node 234 drops, the voltage of a node 242 drops automatically. Thus, since the effective voltage between the base-emitter of the transistor 226 is constant, a change in the static operational current IQ is restricted.
When the operation temperature decreases, the base-emitter turn-on voltage VBE(on) of the transistor 222 increases. However, since the current Imir is almost constantly maintained, the voltage of the node 240 is almost constantly maintained. Thus, the effective voltage between the base-emitter of the transistor 222 decreases so that the collector current of the transistor 222 decreases and the voltage of the node 234 increases. When the voltage of the node 234 increases, the voltage of a node 242 increases automatically. Thus, since the effective voltage between the base-emitter of the transistor 226 is constant, a change in the static operational current IQ is restricted.
In addition, as a conventional technology to finely adjust a voltage applied to diodes of a bias circuit, U.S. Pat. No. 6,566,954 describes an additional compensation function to a temperature compensation function of a bias circuit in which an active device instead of a resistor is inserted in a transistor amplifying an RF signal.
U.S. Pat. No. 6,452,454 describes a technology of an additional temperature compensation function by additionally providing a plurality of diodes in parallel or current paths in the bias circuit to adjust the amount of current flowing from the reference voltage Vref.
U.S. Pat. No. 6,556,082 describes another circuit technology enabling additional temperature compensation function, which is achieved by adding resistors and adjusting ratio between the resistors.
U.S. Pat. No. 6,424,225 describes a technology in which additional circuits are provided to operate according to a change in temperature so that reference current supplied from the bias circuit can be increased or decreased, thus enabling additional temperature compensation in a wider range.